1. Field of the Invention
The present invention relates to a non-magnetic material having high strength and high ductility and, more particularly, to a non-magnetic high manganese cast product which is used in an "as cast" state.
2. Prior Art
In modern technology, various kinds of materials to be used in association with a strong magnetic field have been developed and, in particular, research and development of a non-magnetic materials not influenced by a magnetic field, are very popular. For example, for the purpose of expanding the applicable field of the non-magnetic material to various nuclear fusion reactor equipment, various parts for a magnetic levitation train (linear motor car) and parts for motors and/or transformers, the metallurgical research and development has been promoted widely in the aspects of components of such non-magnetic material as well as heat treatment thereof, and actually a large number of attempts have been heretofore proposed. Reviewing the history of conventional non-magnetic materials, it is understood that an austenitic stainless steel which had been most popularly used as a non-magnetic material has the following disadvantages. That is, in the austenite stainless steel, a large amount of expensive Ni is required and, moreover, transformation may be induced by cold working thereby precipitating a martensite, eventually resulting in a high possibility of deterioration of non-magnetism. Therefore, it is a recent trend that, in place of the mentioned austenitic stainless steel, a non-magnetic high manganese steel has been spotlighted in the art, and a large importance has been increasingly given to research and development of this non-magnetic high manganese steel.
The non-magnetic high manganese steel is advantageous from the economical viewpoint since the same austenitic phase as stainless steel is obtained by substituting any or all of the Ni contained in stainless steel for a cheap Mn and, furthermore, the obtained austenitic phase is stable without transformation induction incidental to cold working, and thus there is less possibility of deterioration occurring in the non-maganetism. On the other hand, the non-magnetic high manganese steel has a disadvantage in that machinability is difficult due to the high percentage of Mn content. To overcome this disadvantage, several attempts have been proposed to expand the applicable range of this material in various uses. An object of such a proposal is directed to improve the machinability of the non-magnetic high manganese steel without affecting or deteriorating its non-magnetism, in other words, to improve ductility for cold and hot rolling as well as to improve steel strength. The machinability as well as steel strength is one of the problems to be solved since this material is directed to be used as a structural material or part of a linear motor car and nuclear fusion reactor. For example, the Japanese Patent Publication (examined) No. 60-54374 discloses a method for producing a cold-rolled austenitic steel plate and steel strip comprising the steps of hot rolling a billet; cold rolling the hot-rolled billet at a rolling percentage of not less than 20%; and annealing the obtained steel at a temperature range of 800.degree.to 1150.degree. C.; the billet containing not more than 0.70% C, not more than 2.5% Si, 9 to 35% Mn, 0.5 to 19.0% Cr, not more than 8% Ni, not more than 0.5% N, not more than 2.0% Al, not more than 0.02% Ca, and the remaining part being composed of iron and unavoidable impuritres. In effect, this Patent Publication proposes a method for producing a steel plate and a steel strip the non-magnetism of which is not deteriorated even when the material is subject to plastic transformation, by defining the rolling and annealing conditions so as to improve stability of the austenitic phase.
The Japanese Patent Publication (examined) No. 60-31897 proposes a specifically deformed non-magnetic reinforcing steel bar the basic elements of which are 0.20 to 1.20% C, 0.10 to 2.0% Si, 5.0 to 35% Mn, 0.50 to 5.0% Ni and 0.20 to 3.0% V, and which contains one or two of not more than 3.0% Cu, not more than 5.0% Cr, not more than 3.0% Mo, not more than 2.0% Ti, not more than 1.0% Zr, not more than 0.30% N, not more than 2.0% Nb, and not more than 2.0% Al. That is, in the non-magnetic steel according to this proposal, the addition of a very small amount of element such as V and others is a required condition, and a hot working is an essential requirement for producing this deformed non-magnetic reinforcing steel bar. As a result of such a structure, it was reported that a deformed non-magnetic reinforcing steel bar of high strength and favorable shearing characteristic was obtained. Further, the Japanese Patent Publication (examined) No. 62-6632 discloses a non-magnetic high manganese steel of improved machinability by adding not only Bi but also Ni, Cr, Al, Nb, V, Ca and S. Furthermore, the Japanese Patent Publication (examined) No. 61-37953 discloses a non-magnetic high manganese steel basically composed of C, Si, Mn, Ni, Cr and N, and of which cold working characteristic and corrosion resistance are improved by hot rolling.
The mentioned non-magnetic high manganese steel according to the prior art intends to improve strength and machinability when used as a structural material, since the material is applied to be a guide way for a magnetic levitation train driven by a linear motor car, a reinforced concrete building for accommodating a nuclear fusion reactor, or a structural member for a generator (dynamo) as mentioned above. In such a conventional way of use, the structural member of non-magnetic high manganese steel incorporated in the mentioned facilities or equipment must be able to bear a heavy load, and to satisfy such a requirement, it is natural that the problem to be solved focusses on the strength and machinability improvement of the obtained non-magnetic steel. Furthermore, the non-magnetic high manganese steel is used to serve as a structural member, and the structural member is usually formed by plastic deformation. Hence there arise a difficult problem of how to prevent transformation induction and, for that purpose, a complicated relation among thermal conditions, restrictions on required components, etc. must be successfully coordinated.
It is, however, to be noted that the industrial field in which non-magnetic steel is used is not limited to the mentioned conventional structural members. Rather, there are now a lot of oppotunities in which non-magnetic steel is used as a functional material. Accordingly, it will be easily understood by persons skilled in the art that different kinds of problems to be solved may arise depending upon the different ways the steel is to be used, and with the progress of technological innovation, yet further problems to be solved may additionally arise.
It is required as a matter of course that, when a material is employed as a member operating under the influence of a strong magnetic field, the material must be a non-magnetic material in order to inhibit as much as possible the generation of heat due to generation of eddy current; in other words, the magnetic permeability .mu. must not be more than 1.05, and furthermore the non-magnetic material serving as a component or a member must have a material strength of a certain level. When further operating conditions are additionally required such that mentioned requirements or properties must be kept unvariable at any part of the member even if the member is large-sized and/or complicated or such that the shape of the member is so intricate that remaining parts which require finishing and/or machining work are difficult, the problems to be solved with regard to such large-sized or complicated non-magnetic material become considerably different from those incidental to the prior art.
For example, in order to prevent heat generation due to the generation of eddy current in the magnetic field, non-magnetic metal fittings such as high strength brass castings, stainless cast steel, etc., have been conventionally employed as a metallic member used for fixing an iron core of a generator. Under the background of recent increasing demand for large-sized generators, the metallic member for fixation of the iron core has been thickened to secure the required strength. There is, however, a restriction on such a thickening of the matallic member for fixation of the iron core due to restrictions on auxiliarly equipment attached to the generator. Non-magnetism is an essential requirement of the metallic member for fixation of the iron core as a matter of course, and furthermore, high strength is likewise required for fixation of the iron core, and high ductility is also required for thermal and mechanical strain of the fixed iron core. Particularly, in the case of a large-size generation, because an absolute quantity of strain tends to increase and become unexpectedly large, the high strength and high ductility of the metallic member for fixation of the iron core become very important properties. Moreover, if the metallic member is large-sized and formed into a complicated shape, a decarburized layer is unavoidably formed on the surface of the material when a solution heat treatment and a water toughening treatment peculiar to the non-magnetic high menganese steel are applied to the metallic member. As a result of this, an unavoidable deterioration of the magnetic permeability is brought about. This decarburized layer is usually removed after heat treatment. However, depending upon the shape of the metallic member, there may be a problem in that the decarburized layer can be neither ground nor machined.